You only need to count the number of revs for 10 secs then multiply by 6 to tell the RPM.

Start the stopwatch as you start to count. If it is too fast to count then it's really moving and you don't have much hope of catching it.

Thanks again, everyone.

Another question, do I have to hide from neutral ships too? I mean, will they alert the enemy?

I've not seen any direct evidence that neutrals report your position and that DD's or aircraft come hunting you if they spot you. but that could be becasue there were no enemy vessels or planes about.

it could be tested in a scripted mission where there is a DD outside sensor range with a neutral close by your sub. Watching the DD to see if they start hunting you when the neutral spots you would confirm whether this happens or not.